1. Field of the Invention
This invention relates to organic photochromic compounds which show photochromism and which are useful as various recording-memory materials or photosensitizers.
2. Description of the Prior Art
A photochromic compound is a compound which shows coloring and discoloring i.e. a "memory" material with reversible photoresponsivity.
The reversible structural changes of photochromic compounds owing to tautomerization, trans-cis isomerization, cyclization, dimerization, ionic dissociation may occur by photoirradiation and also by change of absorption spectrum simultaneously. Since these phenomena (photochromism) are due to changes at the molecular level, the photochromic compound is known as memory material with ultra-high denisty. For example, an expected application is reversible bit-recording material for photodisc, photosensitive material for holograms, or laser recording material. Additionally, owing to its remarkable color changing property the photochromic material with rapid reversible responsivity reaction may be used as an indicator material.
Many inorganic and organic compounds demonstrate photochromism, but in addition to advantages they have problems also.
That is, inorganic photochromic compounds such as Hg.sub.3 S.sub.2 I.sub.2 or ZnS and the like lack workability, abundance of color, and humidity resistance.
On the other hand, various organic photochromic compounds such as azobenzene, spiropyran, viologen, spironaphtooxazine, anthracene, fulgide, stilbene and the like and their derivatives are known. For example, spironaphtohooxazine derivatives and spirobenzopyran are disclosed as photochromic compound in Japanese Patent Publication Nos. 28892/1970, 48631/1974 and Japanese Unexamined Patent Publication No. 36284/1980.
In general, the organic photochromic compound has many advantages such as good workability, the ability of molecular modification to its derivatives to meet to an object, and of formation of various media by dispersion to macromolecular film or covalent crystallization or direct vacuum evaporation to macromolecule, as is desirable of photomemory materials.
However, these known organic photochromic compounds have problems with regard to long term preservability of coloration condition, thermal stability, the repetition character of record (the property relating to the ability of reversible and stable regeneration of record) and the like; thus for this reason they are not used widely now as recording-memory material.